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1971 Larson GM 250 Inline 6 165 HP

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uspoet10

New member
Hi All,

New to forum and relatively new to boating.

Boat:1971 Larson w/ GM 250 inline 6 165 HP. Raw water cooled.

Drive: Pre Alpha

I picked up the boat a couple of months ago and 1st time out overheated. I put in a new impeller kit...including base and upper housing. New thermostat. Still overheated next time out.

I pulled the exhaust manifold and riser. Several passages were completely clogged with rust scale. Cleaned with muriatic acid and flushed. Replaced all hoses. Installed new water shutter...was missing.
Boat can run all day at 3300 RPM but will overheat...gauge in red at 3500.

I was thinking the next step would be to change the circulating water pump. Is that what you guys would do? Any other suggestions? Timing issues? I haven't replaced any ignition parts yet. ANY suggestions are welcome!!!

I'm getting ready to go on vacation (family reunion) and will be gone for 3 weeks so I will have a lot of time to dwell on this problem.

Thanks!
 
Welcome aboard !

You checked for the flapper / shutter in the exhaust pipe ? Can and will block the exhaust.

You checked to see if you have good water at the t-stat ?

Submerge the drive, remove the long hose coming in from the transom at the t-stat housing, hold it straight up and start the engine if you have a 8-10" your good quick shut off the engine you need to do this for a few seconds only.

Say you don't have good water to the t-stat then back flush the long hose see what comes out the drive water pickup.
 
Boat can run all day at 3300 RPM but will overheat...gauge in red at 3500.
Click to expand...
That is a water flow issue. Did you install the thermostat pellet down? Since the exhaust manifold was full of rust you can bet the rest of the engine is in the same condition. Pull the water circulating pump off and have a look. You can flush the engine thru the two openings. Look at the pass thru coupler (remove the long hose) on the gimbal at the inside of the transom where the water inlet hose attaches...may be plugged w/corrosion.
 
Thanks for the reply MM.

The flapper was missing. I put a new one in. I had to cut one of the brackets to get it in. Hopefully it didn't break loose:(.

Are you saying 8-10" at idle? Cooling is fine up to 3300 RPM. Gauge in the green below halfway.

I back flushed after I pulled the exhaust manifold. One of my concerns was that the missing flapper was not intentionally removed and was lodge somewhere. I bought an inspection camera and s***** it in from both directions and did not find anything.

Is is safe to run it above 3500 RPM to check water flow?
 
Thanks for the reply guyjg.

The Thermostat housing only allows me to install the T-stat in 1 direction... spring side down.

The rust problem was evident in the engine. After pulling the T-stat housing and cleaning, I look into the cavity and could see a lot of rust...some scaling at the opening. Plucked out as much as I could. Would this rusting cause insufficient heat transfer and contribute to the overheating at high rpm?

The water circulating pump is on the list for replacement but I won't be able to get to it until the end of the month.

I did notice a slight missing at low RPM's. Like I said in OP I haven't touched any of the ignition components. I haven't even looked at the spark plugs yet. Could timing cause overheating at higher rpms? I'm not a motor guy just your basic DIY'er so please excuse the dumb questions.
 
No question is dumb just have a open mind and like Archie allow us to learn you something.

You have to make sure there is no flapper stuck in the exhaust pipe.

You have to make sure the elbow is open the transfer ports between the mani and elbow are clear.

You have to make sure the impeller in the drive is good and or new with a new housing.

The engine circulation pump can be a auto one but must have ss or bronze blades.

Saw one once that were like razors.

Backwash the hose that feeds the t-stat see what comes out.

Understand grass hopper ?
 
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The engine has had 42 years of rust development. Give all what MM says a shot and see what happens. When you pull off the circulating pump flush out the block w/a garden hose...it may help.
 
Also new boat owner, 1977 silverline with inline6 had only 1hour on engine ran last year overheating replaced impeller took off thermostat housing stat was in upside down. Still overheated
 
check the exhaust elbows?

Any idea if the old (original) manifold has been replaced? The old ones have a cover plate on the intake portion...it usually warps when the engine experiences a good overheat. the newer style does not use the cover plateover the intake portion...
 
Fresh or salt water? Salt water is probably a corroded manifold & riser.

The manifold with the removable top portion went away with early 1970 160 hp models. All 165 models had the 1 piece manifold.(assuming some DIYer didn't retrofit an earlier manifilold).

You replaced the water pump impeller? Did you get the vanes folded the correct direction? In other words, did you turn the drive shaft clockwise looking down as you pushed the housing down over the impeller.

Remove the 5/8" water hose at the t-stat housing & run engine in neutral at dock briefly to verify good water flow out of the hose.

As far as the pump on the front, probably OK if fresh water. Only a handful of bolts to remove to inspect the impeller. New gasket, simple.
 
Silverline 1977 said:
Also new boat owner, 1977 silverline with inline6 had only 1hour on engine ran last year overheating replaced impeller took off thermostat housing stat was in upside down. Still overheated
Click to expand...
I have a question for you.

If you are a new to boating, why would you purchase a boat that is 50 years old for your first boat?

Do you have a solid mechanical background to troubleshoot problems?
Do you have any marine/boat repair experience?
Do you have a service manual to assist in understanding the Marine side of the engine and drive function?

Base engine is basically the same as an Automotive [block and internals and heads and internals] but has a lot of external add ons which make it marine.

Did you lake test the boat before purchase?

I fail to understand why someone who wants to get into boating would start with a 50 year old boat?

Help me understand.......
 
Easy, engine had only 1hour boat was in great shape, local marine shop put in engine ,Ran fine last year I put in new impeller no water so shop put in new impeller have a strong mechanical background put 2nd hose on stat housing still getting hot earlier pulled large hose off stat still had antifreeze in like water circulation pump not pulling stat was totaled bad
 
I find it very strange that one season all is good and the next, complete failure. I also doubt engine had One hour on it. The straight 6 is an antique. Not worth much today. A V6 boat has 40+ more hp and would be more repairable as parts are still readily available. Same with outdrive

My opinion, you have a can of worms on your hands that you will be chasing problems until the day you get rid of it.
 
Copied from your post #12 above:
Ran fine last year I put in new impeller no water so shop put in new impeller have a strong mechanical background...
You put in a new water pump, then no water so a shop put in a new water pump? And you say you have a strong mechanical background?

FYI, the circulating pump circulates what it is sent. IT DOES NOT PULL WATER IN.

Furthermore, this same post #12 is 1 long unpunctuated sentence. So maybe I/we made a mistake trying to understand.

Maybe someone on the Mercury Outboard thread will get you straightened out since you asked over there earlier today. The water flow thru a GM inline 6 is nearly identical to a Mercury outboard.
 
Silverline 1977 said:
Well your sure negative, and the boat did have 1hr I’ll keep trying
Click to expand...
I am not negative, I am realistic.

Hey if you feel its fun to troubleshoot continued problems vs using the boat. Have at it..... Its all good.

I am suggesting you will be fixing more than using..............
 
Droid said:
Copied from your post #12 above:
Ran fine last year I put in new impeller no water so shop put in new impeller have a strong mechanical background...
You put in a new water pump, then no water so a shop put in a new water pump? And you say you have a strong mechanical background?

FYI, the circulating pump circulates what it is sent. IT DOES NOT PULL WATER IN.

Furthermore, this same post #12 is 1 long unpunctuated sentence. So maybe I/we made a mistake trying to understand.

Maybe someone on the Mercury Outboard thread will get you straightened out since you asked over there earlier today. The water flow thru a GM inline 6 is nearly identical to a Mercury outboard.
Click to expand...
Ok thanks it’s my first blog attempt so may not be up to snuff like you all pros
 
Read, reread & consider if you are being clear. If we can't 100% understand the question we can't necessarily give a good answer.
If after you submit a post you decide it needs to be edited, you have 5 minutes to click edit & fix it.

BTW, 1st post or not, good grammar is always nice & helpful.
 
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Silverline 1977 said:
Well I don’t have big money for new boat , it’s fun when it runs I’ll get no help from your negative #&%,
Click to expand...

No one said a NEW boat.

A mid 1980's and newer would be a better choice and there are tens of thousands of them out there. At least that technology level is still very much in use and parts are readily available.
FYI, inline 4 cylinders Mercruisers make 140 HP in newer boats. that inline six at its age may make 150 ish HP
1980's and newer V6 mercruisers range from 190-230+ hp but the torque is far better than the inline 6 Point ignition engines.
These boats are not expensive.

a 50 + year old boat although may not seem expensive now, If you have continued issues it will become so.

At least there several here who's experience goes back that far.
 
uspoet10 said:
Hi All,

New to forum and relatively new to boating.

Boat:1971 Larson w/ GM 250 inline 6 165 HP. Raw water cooled.

Drive: Pre Alpha

I picked up the boat a couple of months ago and 1st time out overheated. I put in a new impeller kit...including base and upper housing. New thermostat. Still overheated next time out.

I pulled the exhaust manifold and riser. Several passages were completely clogged with rust scale. Cleaned with muriatic acid and flushed. Replaced all hoses. Installed new water shutter...was missing.
Boat can run all day at 3300 RPM but will overheat...gauge in red at 3500.

I was thinking the next step would be to change the circulating water pump. Is that what you guys would do? Any other suggestions? Timing issues? I haven't replaced any ignition parts yet. ANY suggestions are welcome!!!

I'm getting ready to go on vacation (family reunion) and will be gone for 3 weeks so I will have a lot of time to dwell on this problem.

Thanks!
Click to expand...
Late ignition timing can cause over heating so for sure verify it is not retarded. If you exhaust was that clogged up willing to bet the block is too
 
I imported this from his Mercury outboard thread:

"1977 silverline 17ft . Wants to stay at 153 degrees @crusing speed 3000 to 4000 rpm, idel reaches 195 . Changed impeller thermostat engine only has 35 hours , took hose off stat housing only pushing 2” of water at idle, put 2nd hose on stat still when to 153 degrees . Could water circulation pump be bad ? Back plushed line to stat also."

For the record, the OEM Mercruiser t-stat is 143 degrees, not 153. I would like to see higher water flow than 2" especially if hose was held horizontally. But I have no idea if he pulled it out of the clamp along the valve cover & held it straight up 2+ feet. Is this being done in water or on garden hose?

I believe he is saying OK at cruise but heats up at idle. If so, we can disregard timing & look at the water pump impeller, housing & workmanship of the installation.
Chineesium crap?
Vanes curled the wring direction (some or all)?
Reused/leaking gasket?
Impeller plate worn?
Previous overheat that has warped the housing?
Bad grommets on either end of copper tube? Assuming the tube was firmly seated in the upper housing, I doubt the copper tube missed it's target because water flow seems to increase with RPM and I would expect zero water flow. (Of course flow will increase with RPM even if all is OK). Now if the guide tube wasn't installed the lower end may have caught the lower grommet & have it partially blocking the tube.
I'd also check the 5/8" water hose from the bell housing to the 12:00 position for cracks/kinks/holes, basically any damage. Also, be sure the o-ring from drive to bell housing is properly in its groove.
 
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A 143 degree stat will have a 150-160* under normal operation. Today you get 140* stat. I am not sure they still sell a 143*.
Also location of sender may show higher or lower a few degrees.

every boat I work on with a 140* stat runs at about 150 ish at idle and normal running rpms.
After a high speed it will reach 165-170 but will come down when lower rpms are reached

Who knows how the impeller was installed considering the thermostat was upside down.

way to many unkowns and so old who knows what was or was not done in the past
 
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